An electrostatic fluid jet applicator is designed to apply a fluid (e.g., a liquid dye) to a moving substrate (e.g., a fabric) by: (a) selectively charging and recovering some of the fluid droplets continuously ejected from a stationary linear array of orifices affixed transverse to the movement of the substrate, while (b) allowing remaining selectively uncharged droplets to strike the substrate (e.g., thereby forming an image on the substrate).
More particularly, fluid is supplied to a linear array of liquid jet orifices in a single orifice array plate disposed to emit parallel liquid streams. These liquid jets break into corresponding parallel lines of droplets falling downwardly toward the surface of a substrate moving transverse to the linear orifice array. A droplet charging electrode array is disposed so as to create an electrostatic charging zone in the area where droplets are formed (i.e., from the jet streams passing from the orifice plate). Selective charging is achieved by individually controlling the application of charge voltage to each charge electrode which, in turn, is arranged to impart an electrostatic charge only to those droplets formed in the vicinity of that electrode. A downstream catching means generates an electrostatic deflection field which deflects all charged droplets into a catcher where they are typically collected, reprocessed and recycled to a fluid supply tank. In this arrangement, only those droplets which happen not to get charged are permitted to continue falling onto the surface of the substrate.
If an image is to be printed, it may be conventionally stored in an electronic digital memory, in the form of binary-valued picture elements (which are typically referred to as pixels). Pixel size is determined by the spacing of charge electrode elements in the transverse direction, and, longitudinally by the mechanical resolution of a rotary pulse generator (e.g., tachometer), coupled to the movement of the substrate. Typically, but not necessarily, transverse and longitudinal resolution are made equal.
With each tachometer pulse, a new line of transverse image data may be transferred from the memory to an array of individual charge voltage control (i.e., charge driver) circuits, which apply a "print" pulse of zero volts to a particular charge element when a pixel is to be printed, or full charge voltage, (typically 150 volts), when a pixel is to be left blank, as determined by the image data for that element.
The amount of fluid applied to a pixel with each print pulse is determined by the duration of the print pulse. The duration is typically set to be greater than or equal to the mean droplet formation rate, to insure that at least one droplet is available per pixel, and is set to be less than or equal to the tachometer pulse period, to insure sufficient time to deposit the required fluid.
The novel driver circuits of the present invention address a number of now recognized problems in the prior art. For example, prior art fluid jet applicators typically utilize individual high voltage driver circuits to apply charge voltage to each of the individual charge electrode elements. Each of these driver circuits determines the characteristics of the charge signal applied to its associated charge electrode, with such characteristics fixed by the driver circuit component values.
In such applicators, each driver circuit typically includes a high voltage switching device such as a transistor associated with each charge element electrode. Such switching devices are digitally controlled to apply or not apply the charge voltage to the charge electrode element to effect or not effect printing. Practical design constraints for such prior art charge driver circuits has typically led to the use of a charge voltage having positive polarity.
It is now recognized that such prior art techniques have several disadvantages. First, adjustment to charge signal characteristics require component changes at each separately controlled high voltage driver circuit, with one driver circuit required for each charge element electrode (e.g., 144 per inch along the transverse orifice array). Secondly, the prior art has typically utilized a positive charge voltage on the electrodes. In addition, the prior art typically has included no mechanism for detecting short circuits on an individual electrode basis.
Using a positive charge voltage is disadvantageous because if a short circuit occurs (e.g., due to fluid sprayed by a misaligned jet), current flows from the charging electrode to ground. Due to well known electrochemical action, metal will be preferentially removed from the more positive electrode and deposited on the more negative ground, thereby resulting in erosion of the relatively expensive charge electrode.
Advantageously, the present invention solves such prior art problems, in part, by employing a print drive bus which is shared by large numbers of relatively simple high voltage charge element electrode drive circuits. Print pulses (of controlled duration and timing and slew rate) present on the print drive bus are selectively used to gate high voltage to individual charge electrodes. In addition, the present invention includes short circuit detection circuitry to provide an indication of the approximate location of the short along the orifice array.
The driver circuit of the present invention is designed to utilize a negative polarity charge voltage to protect the delicate and costly electrode array from erosion due to the aforementioned short circuit problem. As noted above, the typical prior art driver circuit, in practical effect, requires a positive charge voltage which leads to deplating from an electrode upon the occurrence of a short circuit.
Of major significance, it is also now recognized that since the prior art applicators included no mechanism for adjustably controlling the rising and falling edges of print pulses applied to the charge electrodes, no truly satisfactory control over the phenomena known as the "J-Effect" could be achieved. In contrast, the present invention substantially prevents the "J-Effect" from degrading image quality--even under varying operating conditions (e.g., when operating with a variety of orifice plates having distinct orifice diameters).
The "J-Effect" phenomenon in fluid jet charging may be observed by viewing the array of fluid droplets descending from an orifice plate along the axis of the array while printing. At transition times, the path taken by droplets may resemble the letter "J". The "J-Effect" results in a degraded image quality and produces excessive fluid mist which may short circuit the charge and deflection electrodes.
The "J-Effect" is caused due to the interaction of the electric field of previously charged droplet(s) with droplet(s) currently being charged. For example, as a droplet breaks off, it is either not charged (if printing is to occur) or charged (if deflection and catching is to occur). When the charging voltage is turned off abruptly, the droplet now being charged is closely followed by a second droplet which may not be scheduled to be charged. However, due to the close proximity between these droplets, the charged droplet will impart a partial reverse charge on the next droplet formed.
For example, in the present invention, a negative charging electrode is used. If turned "on", the negative charging electrode will induce a positive charge on the droplet then being formed. Presuming the immediately following droplet is intended to have no charge, the positively charged droplet(s) nevertheless can be expected to impart some reverse (i.e., negative) charge on the next droplet(s) formed. Such negatively charged droplet(s) will deflect somewhat away from the catcher and may even strike the substrate causing degraded image quality and/or may produce a fluid mist and cause electrode short circuits. How pronounced the J-Effect may be will vary depending upon operating conditions. For example, different orifice plates having distinct diameter orifices may experience the J-Effect to varying degrees.
The present invention corrects for the J-Effect in a flexible and adjustable manner heretofore not possible in the fluid jet applicator art. In this regard, the J-Effect produced by different orifice plates may be readily compensated by adjusting the present circuit parameters.
Of course, the present invention also functions to dispose a charged droplet in the vicinity of a subsequent droplet which is to be left uncharged. Thus, for example, a partial reverse charging would be expected on the next subsequently formed droplet. However, in addition, the charge electrode in the present invention is left with a partial voltage still on it during this transition period. The combined or net effect of such events results in a nearly zero charge on the subsequent droplet (rather than the normally expected partial reverse charge). The present invention obtains this effect in a manner which allows for ready adaptation to different operating conditions by adjustably controlling the turn-off transition of charge voltage so that it occurs over a period of one or two times the mean droplet formation period for a given operating condition.
The architecture of the present invention advantageously allows the rate of change of charge voltage to be readily adjusted simultaneously for a large number of charge electrodes. Thus, the present invention permits a wide variation in the type of printing that can be accomplished with a jet applicator system by permitting the rate of change of charge voltage to be adjusted to compensate for variations in the stimulation frequency, different orifice diameters, etc., without the need to redesign/reconstruct all the individual charge driver circuits.
The present invention also rapidly turns "on" the charge voltage to minimize the possibility that a particular droplet may be formed during the transition period and thus result in partial charging of the droplet. A partially charged droplet will not be fully deflected and therefore will result in poor catching. Turn-on time is preferably controllably reduced to just short of the point that: (a) cross-talk to adjacent electrodes become a problem or (b) electromagnetic interference (EMI) becomes excessive. The present invention advantageously allows for independent adjustment of charge voltage turn-on and turn-off rates.